Rotor-mast-tilting apparatus and method for lower flapping loads

ABSTRACT

A method and apparatus for reducing flapping loads imposed on a rotor are disclosed. The method may include flying a rotorcraft comprising an airframe, a rotor, a mast extending to connect the rotor to the airframe, a tilt mechanism, at least one sensor, and a computer system. The computer system may obtain in real time, from the at least one sensor, data characterizing at least one flapping load experienced by the rotor during the flying. Using the data, the computer system may issue at least one command to the tilt mechanism. In response to the command, the tilt mechanism may reorient the mast with respect to the airframe. This reorienting may lower the flapping load experienced by the rotor.

RELATED APPLICATIONS

This application: is a divisional (continuation) of U.S. patentapplication Ser. No. 13/373,414, filed Nov. 14, 2011; which claims thebenefit of U.S. Provisional Patent Application Ser. No. 61/575,196,filed Aug. 17, 2011; both of which are hereby incorporated by reference.

Additionally, this patent application hereby incorporates by referenceU.S. Pat. No. 5,301,900 issued Apr. 12, 1994 to Groen et al., U.S. Pat.No. 1,947,901 issued Feb. 20, 1934 to J. De la Cierva, and U.S. Pat. No.2,352,342 issued Jun. 27, 1944 to H. F. Pitcairn.

RIGHTS OF U.S. GOVERNMENT

This invention was made with Government support under Agreement No.HR0011-06-9-0002 awarded by DARPA. The Government has certain rights inthe invention.

BACKGROUND

The Field of the Invention

This invention relates to rotating wing aircraft (rotorcraft), and, moreparticularly to rotorcraft relying on autorotation of a rotor to providelift.

The Background Art

Rotorcraft rely on a rotating wing to provide lift. In contrast,fixed-wing aircraft rely on air flow over a fixed wing to provide lift.Fixed-wing aircraft must therefore achieve a minimum ground velocity ontakeoff before the lift on the wing is sufficient to overcome the weightof the plane. Fixed-wing aircraft therefore generally require a longrunway along which to accelerate to achieve this minimum velocity andtakeoff.

In contrast, rotorcraft can take off and land vertically or along shortrunways inasmuch as powered rotation of the rotating wing provides theneeded lift. This makes rotorcraft particularly useful for landing inurban locations or undeveloped areas without a proper runway.

The most common rotorcraft in use today are helicopters. A helicoptertypically includes an airframe, housing an engine and passengercompartment, and a rotor, driven by the engine, to provide lift. Forcedrotation of the rotor causes a reactive torque on the airframe.Accordingly, conventional helicopters require either two counterrotating rotors or a tail rotor in order to counteract this reactivetorque.

Another type of rotorcraft is the autogyro. An autogyro aircraft deriveslift from an unpowered, freely rotating rotor comprising two or morerotor blades. The energy to rotate the rotor results from awindmill-like effect of air passing through the underside of the rotor(i.e., autorotation of the rotor). The Bernoulli Effect of the airflowmoving over the rotor blade surface creates lift. The forward movementof the aircraft comes in response to a thrusting engine such as a motordriven propeller mounted fore or aft.

During the early years of aviation, autogyro aircraft were proposed toavoid the problem of aircraft stalling in flight and to reduce the needfor runways. In autogyro aircraft, the relative airspeed of the rotorblades may be controlled or influenced somewhat independent of theforward airspeed of the autogyro, allowing slow ground speed for takeoffand landing, and safety in slow-speed flight.

Various autogyro devices in the past have provided some means to beginrotation of the rotor prior to takeoff (i.e., prerotation). Prerotationmay minimize the takeoff distance down a runway. One type of autogyro isthe “gyrodyne.” Examples of such aircraft are the XV-1 convertiplanetested in 1954 and the Rotodyne built by Fairey Aviation in 1962. Thegyrodyne includes a thrust source providing thrust in a flight directionand a rotor providing autorotative lift at cruising speeds. Jet engineslocated on the tip of each rotor blade provided rotation of the rotorduring takeoff, landing, and hovering.

Although typical rotorcraft provide the significant advantage ofvertical takeoff and landing (VTOL), they are much more limited in theirmaximum flight speed than are fixed-wing aircraft. One reason that priorrotorcraft are unable to achieve high flight speed is a phenomenon knownas “retreating blade stall.”

In a fixed-wing aircraft, all wings move forward in fixed relation withrespect to one another and the airframe. However, as a rotorcraft movesin a flight direction, rotation of the rotor causes each blade thereofto be either “advancing” or “retreating.” A blade is advancing if it ismoving in the same direction as the flight direction. A blade isretreating if it is moving opposite the flight direction. Thus, thevelocity of any point on any blade is the velocity of that point, withrespect to the airframe, plus the velocity of the airframe.

Rotor blades are airfoils that provide lift based on the speed of airflow thereover. Accordingly, the advancing blade typically experiencesmuch greater lift than the retreating blade. If left uncheck, thisdisproportionate lift may render the rotorcraft unflyable. One solutionto this problem is allowing the rotor blades to “flap.” Flapping enablesrotorcraft to travel in a direction substantially perpendicular to theaxis of rotation of the rotor.

With flapping, an advancing blade is allowed to fly or flap upward inresponse to the increased air speed thereover, thereby reducing theblade's angle of attack. This, in turn, reduces the lift generated bythe advancing blade. A retreating blade experiences less air speed andtends to fly or flap downward such that its angle of attack isincreased. This, in turn, increases the lift generated by the retreatingblade. In this manner, flapping balances the lift generated by theadvancing and retreating blades.

However, lift equalization due to flapping is limited by retreatingblade stall. As noted above, flapping of the rotor blades increases theangle of attack of the retreating blade. At certain higher speeds in thedirection of flight, the increase in the blade angle of attack requiredto equalize lift results in loss of lift (stalling) of the retreatingblade.

A second limit on the speed of rotorcraft is the drag at the tips of therotor blades. The tip of the advancing blade is moving at a speed equalto the speed of the aircraft relative to the surrounding air, plus thespeed of the tip of the blade with respect to the aircraft. Thus, thespeed at the tip of an advancing blade is equal to the sum of the flightspeed of the rotorcraft plus the product of the length of the blade andthe angular velocity of the rotor.

In helicopters, the rotor must rotate to provide both upward lift andthrust in the direction of flight. Increasing the speed of a helicopterincreases the air speed at the tip, both because of the increased flightspeed as well as the increased angular velocity of the rotors requiredto provide supporting thrust. The speed at the tip of the advancingblade could therefore approach the speed of sound, even when the flightspeed of the rotorcraft was actually much less. As the air speed overthe tip approaches the speed of sound, the drag on the blade becomesgreater than the engine can overcome. Accordingly, helicopters are quitelimited in how fast they can fly.

In autogyro aircraft, the tips of the advancing blades are also subjectto this increased drag, even for flight speeds much lower than the speedof sound. The tip speed for an autogyro is typically smaller than thatof a helicopter, for a given airspeed, since the rotor is not driven.Nevertheless, the same drag increase occurs eventually.

A third limit on the speed of rotorcraft is reverse air flow over theretreating blade. As noted above, the retreating blade is travelingopposite the flight direction with respect to the airframe. At certainhigh speeds in the direction of flight, portions of the retreating blademay move rearward, with respect to the airframe, slower than the flightspeed of the airframe. Accordingly, the direction of air flow over thoseportions of the retreating blade is reversed from that typicallydesigned to generate positive lift.

Rather then generating positive lift, reverse air flow may imposenegative lift, or a downward force, on the retreating blade. That is, anairfoil with positive angle of attack in a first direction has anegative angle of attack in a second direction, opposite the firstdirection.

The ratio of air speed of a rotorcraft in the direction of flight to themaximum corresponding air speed at the tips of the rotor blades is knownas the “advance ratio.” The maximum advance ratio of currently availablerotorcraft is less than 0.5. For most helicopters, the maximumachievable advance ratio is between about 0.3 and 0.4. Accordingly,current rotorcraft are limited to a top flight speed of about 200 milesper hour (mph) or less.

In view of the foregoing, it would be an advancement in the art toprovide a rotorcraft capable of flight speeds well in excess of 200 mph.

BRIEF SUMMARY OF THE INVENTION

The invention has been developed in response to the present state of theart and, in particular, in response to the problems and needs in the artthat have not yet been fully solved by currently available apparatus andmethods. The features and advantages of the invention will become morefully apparent from the following description and appended claims, ormay be learned by practice of the invention as set forth hereinafter.

Flapping (i.e., bending of a rotor blade up and down) may causerelatively high moments and attendant stresses in a stiff blade-rootattachment. Additionally, flapping may also induce relatively highchord-wise loads due to Coriolis forces. These loads may quickly fatiguea rotor. Accordingly, in embodiments in accordance with the presentinvention, systems may be implemented to limit or minimize flappingloads imposed on a rotor under at least all steady state, and preferablyall, flight conditions.

Additionally, a rotor in accordance with the present invention mayoperate at a wide range of rotational speeds. One or more naturalfrequencies or harmonic regions corresponding to a rotor may becontained within that range. During the crossing of any such naturalfrequency, the dynamic response of a rotor to periodic loads mayamplified, leading to elevated structural loads and potentially highvibrations. Accordingly, in embodiments in accordance with the presentinvention, systems may be implemented to minimize the time needed forsuch natural frequency crossings.

In selected embodiments, to minimize flapping loads and the timerequired to cross a natural frequency, a rotorcraft in accordance withthe present invention may include a tilt mechanism permitting in-flight,real time adjustments to the orientation of a mast with respect toairframe. For example, in selected embodiments, wing incidence may befixed and selected to allow both the wings and the rest of the airframeto operate at their best lift-to-drag ratio. However, a tilt mechanismmay enable a rotor to move (e.g., tilt, reorient, etc.) with respect toan oncoming airflow and provide another control over rotor disk angle ofattack, independent of collective and cyclic pitch controls.

A tilt mechanism may permit or support only one degree of freedom. Forexample, a tilt mechanism may support exclusively pivoting of a mastabout a laterally extending axis. This may enable a mast (andcorresponding rotor) to pitch fore and aft. Alternatively, a tiltmechanism may permit or support more than one degree of freedom. Forexample, a tilt mechanism may exclusively support pivoting of a mastabout a laterally extending axis and about a longitudinally extendingaxis. This may enable a mast to pitch fore and aft and roll left andright.

In selected embodiments, a tilt mechanism may form part of a controlsystem. For example, in operation, a rotor may need to assume an angleof attack that varies with aircraft gross weight, speed, rotationalspeed of the rotor, air density, air temperature, and the like. Byincluding a tilt mechanism within a control system, potentially constantchanges may be made to the orientation of a mast in order to optimizeselected functions, performance, or the like of the correspondingrotorcraft.

For example, in selected embodiments, a control system may include oneor more sensors outputting signal characterizing one or more flappingloads being experienced by a rotor. This signal may be fed into anaeromechanics model of the rotorcraft running on an onboard computersystem. The aeromechanics model may be solved in real time by thecomputer system. Based on the outputs of the model, the computer systemmay issue commands to various actuators. For example, the computersystem may issue one or more commands to a tilt mechanism. Acting on thecommands, the tilt mechanism may reorient a corresponding mast, pivotingit from first position to a second position. The second position maycorrespond to lower flapping loads, given current flight conditions.

Alternatively, or in addition thereto, a control system may beconfigured to minimize the time needed for crossing certain naturalfrequencies of a rotor. In selected embodiments, a control system mayinclude and manipulate controls over rotor longitudinal and lateralcyclic pitch, rotor collective pitch, rotor mast longitudinal tilt,rotor mast lateral tilt, fixed wing pitch, roll, and yaw controls (e.g.,elevator, ailerons, rudder), rotorcraft thrust, or the like orcombinations or sub-combinations thereof.

For example, during flight conditions when the rotational speed of arotor is removed by a proper separation from a critical harmonic, acomputer system running a aeromechanics model may continuously sets allavailable controls to achieve a best overall lift-to-drag ratio andminimize flapping. However, as a pilot transitions a rotorcraft througha critical harmonic, the aeromechanics model may temporarily transitionfrom a first mode corresponding to best overall lift-to-drag ratio to asecond mode corresponding to fastest change in rotational speed of therotor.

In a second mode effecting a decrease in rotational speed of a rotor, arotor may be tilted further forward than required for steady stateautorotation. The result may be a rapid slowing in the rotational speedof the rotor to a value desired at the airspeed above the criticalfrequency crossing. In a second mode effecting an increase in rotationalspeed of a rotor, a rotor may be tilted further aft than required forsteady state autorotation. The result may be a rapid increase in therotational speed of the rotor to a value desired at the airspeed belowthe critical frequency crossing. Once a crossing has been made and aproper separation has been established, an aeromechanics model maytransition back to a first mode corresponding to best overalllift-to-drag ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of the present invention will become more fullyapparent from the following description and appended claims, taken inconjunction with the accompanying drawings. Understanding that thesedrawings depict only typical embodiments of the invention and are,therefore, not to be considered limiting of its scope, the inventionwill be described with additional specificity and detail through use ofthe accompanying drawings in which:

FIG. 1 is a perspective view of a rotorcraft in accordance with oneembodiment of the present invention, the rotorcraft having two enginesand one rotor;

FIG. 2 is a schematic front elevation view of a compressed or otherwisepressurized air supply for tip jets in accordance with one embodiment ofthe present invention;

FIG. 3A is a front elevation view of a rotorcraft illustratingoperational parameters describing a rotor configuration suitable for usein accordance with the present invention and the system of FIGS. 1 and 2in particular;

FIG. 3B is a right side elevation view of the rotorcraft of FIG. 3A;

FIG. 3C is a partial cut of a right side elevation view of the rotor ofFIG. 3A;

FIG. 4 is a right side elevation view of a rotorcraft illustratingoperational parameters describing a rotor mast pivoting about an axisextending laterally in accordance with the present invention;

FIG. 5 is a front elevation view of a rotorcraft illustratingoperational parameters describing a rotor mast pivoting about an axisextending longitudinally in accordance with the present invention;

FIG. 6 is a schematic diagram of one embodiment of a control system inaccordance with the present invention;

FIG. 7 is a schematic diagram of one embodiment of a control system forcontrolling the orientation of a rotor mast in accordance with thepresent invention;

FIG. 8 is a graph illustrating various natural frequencies or harmonicsthat may be encountered within the operating envelope of a rotor inaccordance with the present invention;

FIG. 9 is a partial perspective view of one embodiment of a rotor, mast,and tilt mechanism in accordance with the present invention;

FIG. 10 is a cross-sectional view of the rotor, mast, and tilt mechanismof FIG. 9;

FIG. 11 is a partial perspective view of the mast and tilt mechanism ofFIG. 9;

FIG. 12 is another partial perspective view of the mast and tiltmechanism of FIG. 9; and

FIG. 13 is a partial side view of one embodiment of a tilt mechanismsupporting pivoting of a rotor mast about two independent, orthogonalaxes.

DETAILED DESCRIPTION OF SELECTED EMBODIMENTS

It will be readily understood that the components of the presentinvention, as generally described and illustrated in the drawingsherein, could be arranged and designed in a wide variety of differentconfigurations. Thus, the following more detailed description of theembodiments of the system and method of the present invention, asrepresented in the drawings, is not intended to limit the scope of theinvention, as claimed, but is merely representative of variousembodiments of the invention. The illustrated embodiments of theinvention will be best understood by reference to the drawings, whereinlike parts are designated by like numerals throughout.

Referring to FIG. 1, a rotorcraft 10 in accordance with the presentinvention may include an airframe 12 defining a cabin for carrying anoperator, passengers, cargo, or the like. The airframe 12 may includeone or more fixed wings 14 or airfoils 14 providing lift to therotorcraft 10. The wings 14 may be configured such that they providesufficient lift to overcome the weight of the rotorcraft 10 (or anysignificant portion thereof) only at comparatively high speeds.

That is, a rotorcraft 10 may be capable of vertical takeoff and landing(VTOL) and may not need lift from the fixed wings 14 at low speeds(e.g., below 50 mph or even 100 mph). Accordingly, the wings 14 may bemade smaller than those of fixed-wing aircraft requiring a high velocitytakeoff. The smaller wings 14 may result in lower drag at highervelocities. In some embodiments, the wings 14 may provide sufficientlift to support at least 50 percent, preferably about 90 percent, of theweight of the rotorcraft 10 at air speeds above 200 mph.

Control surfaces 16 may form part of an airframe 12. For example a tailstructure 18 may include one or more vertical stabilizers 20 and one ormore rudders 22. The rudders 22 may be adjustable to control yaw 24 ofthe rotorcraft 10 during flight. As known in the art, yaw 24 is definedas rotation about a vertical axis 26 of the rotorcraft 10. In theillustrated embodiment, the rudders 22 may comprise hinged portions ofthe vertical stabilizers 20.

The tail structure 18 may further include a horizontal stabilizer 28 andan elevator 30. The elevator 30 may be adjustable to alter pitch 32 ofthe rotorcraft 10. As known in the art, pitch 32 is defined as rotationabout an axis extending laterally with respect to the airframe 10. Inthe illustrated embodiment, the elevator 30 is a hinged portion of thehorizontal stabilizer 28. In some embodiments, twin rudders 22 may bepositioned at an angle relative to the vertical axis 26 and serve bothto adjust or control yaw 24 and pitch 32 of the rotorcraft 10.

The control surfaces 16 may also include ailerons 36 on the wings 14.Ailerons 36 may be used to control roll 38 of the rotorcraft 10. Asknown in the art, roll 38 is defined as rotation about the longitudinalaxis 34 of the rotorcraft 10.

Lift during vertical takeoff and landing, and for augmenting lift of thewings 14 during flight, may be provided by a rotor 40. A rotor 40 maycomprise a number of individual rotor blades 42 extending radially awayfrom a hub 44. The hub 44 may be coupled to a mast 46. The mast 46 mayextend to connect the hub 44 to the rest of the airframe 12.

Referring to FIG. 2, a rotor 40 may be coupled to one or more engines 48housed in a fuselage portion of the airframe 12 or in one or moreadjacent nacelles. The engines 48 may provide thrust during flight ofthe rotorcraft 10. The engines 48 may also generate compressed air forthe tip jets 50.

For example, in selected embodiments, the engines 48 may comprise one ormore bypass turbines 62. All or a portion of the bypass air from theturbines 62 may be directed to the tip jets 50. Alternatively, theengines 48 may drive one or more auxiliary compressors, which in turnmay provide the compressed air for the tip jets 50. In still otherembodiments, all or a portion of the compressed air may be generated byone or more dedicated, single purpose engines, motors, or the like.Using the compressed air, the tip jets 50 may power the rotor 40 duringtakeoff, landing, hover, or whenever the flight speed of the rotorcraft10 is too low for sufficient lift from autorotation of the rotor 40.

In selected embodiments, the compressed air generated by the engines 48may be conducted to the tip jets 50 via one or more conduits or ducts54, 55. For example, bypass air from one or more bypass turbines 62 maybe transmitted through ducts 54 to a plenum 56. The plenum 56 may be influid communication via ducting 55 with a mast 46 that is hollow or hasanother passage to provide for air conduction. For example, a mastfairing 58 positioned around the mast 46 may provide one or both of anair channel and a low drag profile for the mast 46. The mast 46 or mastfairing 58 may be in fluid communication with a hub 44. Finally, the hub44 may be in fluid communication with an interior conduit 60 within eachof the various rotor blades 42. Accordingly, the compressed air maytravel radially within the interior conduits 60 to feed thecorresponding tip jets 50.

Referring to FIGS. 3A-3C, rotation of the rotor 40 about its axis ofrotation occurs in a rotor disc 70 that is generally planar but may becontoured due to flexing of the blades 42. In general, the rotor disc 70may be defined as a space in which the tips of the blades 42 travel.Inasmuch as the blades 42 flap cyclically upward and downward due tochanges in lift while advancing and retreating, the rotor disc 70 may beangled or contoured with respect to the axis of rotation when viewedalong the longitudinal axis 34, as shown in FIG. 3A.

Referring to FIG. 3B, the angle 74 of the rotor disc 70, relative to aflight direction 76 in the plane containing the longitudinal axis 34 andvertical axis 26, is defined as the rotor angle of attack 74 or rotordisk angle of attack 74. For purposes of this application, flightdirection 76 and air speed refer to the direction and speed,respectively, of the airframe 12 of the rotorcraft 10 relative tosurrounding air. In autogyro systems, the angle of attack 74 of therotor disc 70 is generally positive in order to achieve autorotation ofthe rotor 40 and the resulting lift.

Referring to FIG. 3C, the surfaces of the rotor blades 42, andparticularly the chord of each blade 42, define a pitch angle 78, orblade angle of attack 78, relative to the direction of movement 80 ofthe rotor blades 42. In general, a higher pitch angle 78 will result inmore lift and higher drag on the rotor blade 42, up to the point wherestalling occurs (at which point lift has declined below a valuenecessary to sustain flight). The pitch angle 78 of the rotor blade 42may be manipulated by both cyclic and collective pitch controls.

Herein, a trailing letter on a reference numeral indicates a specificinstance of the generic item indicated by the reference numeral. Thusreference may be made in the text to a generic item corresponding to areference numeral, without using a trailing reference letter, eventhough that numeral is never shown in the drawings without such atrailing letter.

Referring to FIGS. 4 and 5, in selected embodiments, a rotorcraft 10 inaccordance with the present invention may be suitable for cruise speedsup to 400 mph and above. Above aircraft airspeeds of approximately 40mph, a rotorcraft 10 may operate as a gyroplane (i.e., an aircraft inwhich the rotor 40 is driven by external aerodynamic forces only;windmilling) No shaft torque or tip jet thrust need be provided at suchspeeds. Accordingly, the rotor 40 may be said to be in sustainedautorotation.

As long as the rotational speed of a rotor 40 is maintained at aconstant value or increased, the airspeed of an advancing blade tipincreases with aircraft airspeed, i.e, forward airspeed relative to thesurrounding air. Accordingly, it may be necessary for efficiency andother reasons to slow down the rotational speed of the rotor 40 duringcertain flight conditions (e.g., during high speed flight or flight athigh advance ratios). For example, it may be necessary to maintain theairspeed of an advancing blade tip at approximately Mach 0.9 or below.

That is, at low aircraft airspeeds, the rotational speed of a rotor 40must be maintained at a sufficiently high value to allow the liftprovided by the rotor 40 to support the majority of the weight of therotorcraft 10. However, as aircraft airspeed increases, an advancingblade 42 will reach a Mach number at which drag divergence occurs withits attendant performance penalties. Consequently, as aircraft airspeedincreases further, this penalty can only be avoided by slowing therotational speed of a rotor 40. In certain embodiments, this maycomprise slowing the rotational speed of a rotor 40 to about fortypercent of the rotational speed corresponding to conventionalautorotative flight (e.g., autorotative flight somewhere in the range ofabout 50 mph to about 100 mph).

A slowed rotor 40, however, may produce less lift, causing more of theweight of the rotorcraft 10 to be transferred to one or more fixed wings14. Thus, sustained autorotation over the entire gyroplane speed rangeof a rotorcraft 10 may require appropriate manipulation of a number ofcontrols on the rotor 40 and the fixed wings 14 to achieve an optimizedrotorcraft system that is aerodynamic efficient and maintain loadsimposed on a rotor 40 within acceptable limits.

The blades 42 and hub 44 of a rotor 40 operating at advance ratios wellabove a value of two may need to meet certain stiffness requirements.For example, to maintain flapping stability, it may be necessary tomaintain a high flap-wise stiffness and a high flapping inertia. It mayalso be highly desirable to maintain the ratio of a first (lowestfundamental) in-plane natural frequency to rotational cyclic rate of therotor 40 well above one. Such a rotor 40 may be characterized as“in-plane stiff,” where the plane is defined by the rotor disk.

A rotor 40 that is in-plane stiff may avoid crossing a first in-planenatural frequency as the rotor 40 is slowed. This may be desirablebecause crossing this frequency may easily lead to another instabilityreferred to as air resonance. However, a rotor 40 that is in-plane stiffmay still experience two relatively high bending loads.

Specifically, flapping (i.e., bending of a blade 42 up and down) maycause relatively high moments and attendant stresses in a stiffblade-root attachment. Additionally, flapping may also induce relativelyhigh chord-wise loads due to Coriolis forces. These loads may quicklyfatigue a rotor 40. Accordingly, in embodiments in accordance with thepresent invention, systems may be implemented to limit or minimizeflapping loads imposed on a rotor 40 under at least all steady state,and preferably all, flight conditions.

The structures and control systems of a rotorcraft 10 in accordance withthe present invention may be configured to maintain or meet one or moreconditions. For example, a rotorcraft 10 may be configured to maintain arotor 40 in sustained autorotation and at a controlled rotational speed.This may be accomplished at different rotor-lifting levels, each ofwhich may correspond to a different rotor angle of attack 74 (disk angle74 or disk angle of attack 74). In selected embodiments, this rotorangle of attack 74 may be determined at least partially by a combinationof collective and cyclic pitch settings.

A rotorcraft 10 may be further configured to share or distribute weightbetween one or more rotors 40 and one or more fixed wings 14 to achievean optimized overall lift-to-drag ratio. In selected embodiments orsituations (e.g., high speed flight or long range cruising), both arotor 40 and fixed wing 14 may be lifting with a minimized combineddrag. In certain embodiments, this may be accomplished at least in partby maintaining low trim drag on various control surfaces (e.g.,horizontal stabilizers 30 or elevators 30).

A rotorcraft 10 may be further configured to maintain the loads imposedon a rotor 40 within design limits. In selected embodiments andsituations, this may be accomplished by minimizing flapping loads andmaintaining a rotor outside of any higher harmonic resonance (i.e.,1/rev. may be automatically avoided by an in-plane stiff design). Tominimize flapping loads, a rotorcraft 10 in accordance with the presentinvention may include a tilt mechanism 82 permitting in-flight, realtime adjustments to the orientation of a mast 46 with respect to theairframe 12.

For example, in selected embodiments, wing incidence (angle of attackwith respect to the airframe or fuselage) may be fixed and selected toallow both the wings 14 and the rest of the airframe 12 (e.g., fuselage)to operate at their best lift-to-drag ratio. However, a tilt mechanism82 may enable a rotor 40 to move (e.g., tilt, reorient, etc.) withrespect to an oncoming airflow and provide another control over rotordisk angle of attack 74, independent of collective and cyclic pitchcontrols.

In selected embodiments, a tilt mechanism 82 may permit or support onlyone degree of freedom. For example, a tilt mechanism 82 may supportexclusively pivoting of a mast 46 about a laterally extending axis. Thismay enable a mast 46 (and corresponding rotor 40) to pitch fore 84 andaft 86. In certain embodiments, a tilt mechanism 82 may support a mast46 in a nominally “neutral” position. The neutral position maycorrespond to a particular rotor disk angle of attack 74. From theneutral position, a tilt mechanism 82 may pitch a mast 46 fore 84 todecrease the rotor disk angle of attack 74 and aft 86 to increase therotor disk angle of attack 74.

In selected embodiments, the maximum rotor disk angle of attack 74imposed by a tilt mechanism may be about ten degrees or less.Conversely, the minimum rotor disk angle of attack 74 imposed by a tiltmechanism 82 may be from about zero to about negative four degrees, orthe like. In selected embodiments, the range of motion of a mast 46 froman angle of attack 74 of about zero degrees to an angle of attack 74 ofabout negative four degrees may be accessed or utilized exclusivelywhile a rotor 40 is being powered (e.g., by tip jets 50).

In certain embodiments, a tilt mechanism 82 may permit or support morethan one degree of freedom. For example, a tilt mechanism 82 mayexclusively support pivoting of a mast 46 about a laterally extendingaxis and about a longitudinally extending axis. This may enable a mast46 (and corresponding rotor 40) to pitch fore 84 and aft 86 and rollleft 88 and right 90. Rolling left 88 and right 90 may enable a tiltmechanism 82 to limit, reduce, or minimize certain forces imposed on arotor 40 to a degree that cannot fully be addressed by pitching fore 84and aft 86. In general, the magnitude of the rolling left 88 and right50 permitted or provided by a tilt mechanism 82 may be small compared tothe magnitude of the pitching fore 84 and aft 86. In selectedembodiments, the maximum roll to the left 88 or right 90 may be aboutfive degrees or less.

Referring to FIG. 6, in selected embodiments, a rotorcraft 10 mayinclude one or more control systems 92. In certain embodiments, acontrol system 92 may include one or more sensors 94, one or moreindependent or interconnected computer systems 96 or controllers 96, oneor more actuators 98, and one or more flight-control components 100.Suitable sensors 94 may include strain gauges, accelerometers, airspeedsensors, rotational speed or cycle frequency sensors, and the like. Acomputer system 96 may receive and interpret a signal from the one ormore sensors 94 and issue commands based, at least in part, on thesignal or information learned therefrom.

In certain embodiments, a computer system 96 may comprise or run anaeromechanics model of the rotorcraft 10. Inputs corresponding to orcomprising signals from the one or more sensors 64 may be fed into themodel. The aeromechanics model equations may be solved onboard in realtime. Appropriate responses to such inputs may then be relayed by acomputer system 96 to one or more actuators 98. Acting on commandsreceived from a computer system 96, the actuators 98 may manipulatecorresponding flight-control components 100, which in turn may affectthe manner in which the corresponding rotorcraft 10 flies.

Referring to FIG. 7, in selected embodiments, a tilt mechanism 82 mayform part of a control system 92. For example, a tilt mechanism 82 maycomprise one or more actuators 98 and flight control components 100within an automatically variable mast tilt (AVMT) system 92 ordynamically variable mast tilt (DVMT) system 92. In operation, a rotor40 may need to assume an angle of attack 74 that varies with aircraftgross weight, speed, rotational speed of the rotor 40, air density, airtemperature, and the like. By including a tilt mechanism 82 within anAVMT system 92, potentially constant changes may be made to theorientation of a mast 46 in order to optimize selected functions,performance, or the like of the corresponding rotorcraft 10.

An AVMT system 92 may include one or more sensors outputting a signalcharacterizing one or more flapping loads being experienced by a rotor14. This signal may be fed into an aeromechanics model of the rotorcraft10 running on an onboard computer system 96. The equations of theaeromechanics model may be solved in real time by the computer system96. Based on the outputs of the model, the computer system 96 may issuecommands to various actuators 98.

For example, the computer system 96 may issue one or more commands to atilt mechanism 82. Acting on the commands, the tilt mechanism 82 mayreorient a corresponding mast 46, pivoting it from one position toanother position. The computer system 96 may also issue one or morecomplementary commands. For example, the computer system 96 may issueone or more commands to an actuator 98 corresponding to an elevator 30.Accordingly, multiple flight-control components 100 may be adjustedsimultaneous, sequentially, or as needed to optimize a selectedfunction, optimize aerodynamic performance, contain or minimize astructural load (e.g., flapping load), or the like or combinations orsub-combinations thereof. Thus, in selected embodiments, an AVMT system92 may enable a rotorcraft 10 in accordance with the present inventionto maintain flapping loads near or substantially at zero and control therotational speed (radians per second) or its corresponding equivalentrotational frequency (cycles per second) of a rotor 40 not only incruise flight, but also during dynamic rotorcraft maneuvers.

Referring to FIG. 8, in selected embodiments, a tilt mechanism 82 inaccordance with the present invention may be used to limit, contain, orcontrol the dynamic response of rotor blades 42 when traversing naturalfrequencies of a rotor 40. That is, as a rotor 40 transitions from arelatively higher rotational speed (or corresponding cyclic frequency),needed for low speed flight, to a relatively lower rotational speed,needed for high speed flight, and then back again, the rotor 40 musttypically cross at least one natural frequency of the rotor blade.Accordingly, in selected embodiments, a tilt mechanism 82 may beconfigured and operated to mitigate such crossings.

As stated above, to maintain flapping stability, it may be necessary tomaintain a high flap-wise stiffness as well as a high flapping inertia.This may be due to the fact that the centrifugal stiffening of a rotor40 operating at forty percent of nominal rotational speed (e.g., therelatively high rotational speed that may be associated withconventional sustained autorotative flight) may have the benefit of onlyabout sixteen percent of the normal centrifugal stiffening. Highflap-wise stiffness, on the other hand, may reduce the amount offlapping a rotor 40 can sustain within structural fatigue limits (e.g.,reduce permitted flapping deflection to something on the order of abouttwo degrees). This amount of flapping may be insufficient to positionthe rotor tip path plane relative to the fuselage and wing system andthe free stream of air for optimal trim efficiency and during maneuvers.A rotorcraft 10 in accordance with the present invention may include anAVMT system 92 to overcome this constraint.

A rotor 40 that is in-plane stiff may avoid crossing a first in-planenatural resonance mode, which has very low damping, as the rotor 40slows. Since a rate of 1/revolution excitations tend to be large,crossing this frequency could easily lead to unacceptably highstructural loads. A rotor 40 in accordance with the present inventionmay be in-plane stiff to avoid this frequency. However, when therotational speed of a rotor 40 decreases in flight by as much as sixtypercent, other natural frequencies with higher harmonic excitations maybe unavoidable.

FIG. 8 provides a typical frequency diagram 102 for a rotor 40 that isin-plane stiff. The diagram 102 illustrates three natural frequencycrossings 104 a, 104 b, 104 c or crossing regions 104 a, 104 b, 104 cwithin the range 106 of rotational speeds to be experienced by a rotor40. During any such crossing 104 a, 104 b, 104 c or region 104 a, 104 b,104 c of a natural frequency, the dynamic response of a rotor 40 toperiodic loads may amplified, leading to elevated structural loads andpotentially high vibrations.

Accordingly, it may be desirable to have a control system 92 minimizethe time needed for such natural frequency crossing 104 a, 104 b, 104 c.Such a control system 92 may execute the crossing 104 a, 104 b, 104 cusing exclusively aerodynamic forces, since the rotor 40 may not bepowered through the shaft or any reaction drive device (e.g., tip jet 50or the like).

In operation, as a rotorcraft 10 increases speed (e.g., using jet orpropeller thrust) to achieve cruise speed, the rotational speed of arotor 40 may be slowed in a smooth and continuous fashion up to a pointwhen a natural frequency is impending. At any such point, the rotationalspeed of the rotor 40 may be “stepped down” to below the frequencycrossing 104 a, 104 b, 104 c with a proper separation therefrom (e.g., aseparation of about three percent). The same may be true, but inreverse, during deceleration of the rotorcraft 10.

To make this happen, a rotorcraft 10 may remain trimmed and not exhibitany objectionable pitch, roll, or yaw oscillations while the rotationalspeed of a rotor 40 is stepped down or up. The rotational speed of therotor 40 may be decreased or increased in minimum time. Additionally,flapping loads may be kept to a minimum.

A control system 92 configured to minimize the time needed for crossing104 a, 104 b, 104 c a natural frequency may include and manipulatecontrols 100 over rotor longitudinal and lateral cyclic pitch, rotorcollective pitch, rotor mast longitudinal tilt, rotor mast lateral tilt,fixed wing pitch, roll, and yaw controls (e.g., elevator 30, ailerons36, rudder 22), rotorcraft thrust, or the like or combinations orsub-combinations thereof.

During flight conditions when the rotational speed of a rotor 40 orcorresponding cyclic frequency is removed by a proper separation (e.g.,a three percent separation) from a critical harmonic, a computer system96 running an aeromechanics model may continuously set all availablecontrols to achieve a best overall lift-to-drag ratio and minimizeflapping. However, as a pilot transitions a rotorcraft 10 through acritical harmonic, the aeromechanics model may temporarily transitionfrom a first mode corresponding to best overall lift-to-drag ratio to asecond mode corresponding to a fastest change in rotational speed of therotor 40.

In selected embodiments, in this second mode or during a transitionthereto, a computer system 96 may pre-compute a desired steady statetrim for the airspeed and rotational speed of a rotor 40 correspondingto a proper separation from the natural frequency or frequencies. In asecond mode effecting a decrease in rotational speed of a rotor 40(i.e., an acceleration of the rotorcraft 10), a rotor 40 may be tiltedfurther forward 84 than required for steady state autorotation.Additionally, such a second mode may mandate operating one or moreengines 48 at maximum thrust to make the crossing 104 a, 104 b, 104 c asquickly as possible. The result may be a rapid slowing in the rotationalspeed of the rotor 40 to a value desired at the airspeed above thecritical frequency crossing 104 a, 104 b, 104 c that corresponds to aproper frequency separation. This method of transition may becharacterized as a dynamic transition.

In a second mode effecting an increase in rotational speed of a rotor 40(i.e., a deceleration of the rotorcraft 10), a rotor 40 may be tiltedfurther aft 86 than required for steady-state autorotation.Additionally, such a second mode may mandate operating one or moreengines 48 at minimum thrust to make the crossing 104 a, 104 b, 104 c asquickly as possible. The result may be a rapid increase in therotational speed of the rotor 40 to a value desired at the airspeedbelow the critical frequency crossing 104 a, 104 b, 104 c thatcorresponds to a proper frequency separation.

For a dynamic transition, it may be necessary for a computer system 96to predict the entire transition sequence, including the associatedairspeed change and change in lift sharing between a rotor 40 and wing14 for the flight condition of the rotorcraft 10 at the time. Anaccurate knowledge of the gross weight of the rotorcraft 10 may beimportant. Considering the need for the lift provided by the rotor 40and the lift provided by the wing 14 to change independently andsimultaneously, keeping flapping loads within allowable limits may alsobe important. As lateral-tip-path-plane motions may occur during theprecession of the tip path plane, a tilt mechanism 82 may provide orsupport tilting left 88, tilting right 90, or some combination thereof.

Once a crossing 104 a, 104 b, 104 c has been made and a properseparation has been established, an aeromechanics model may transitionback to a first mode corresponding to obtaining the best overalllift-to-drag ratio. A proper separation from the rotational speedcorresponding to a natural frequency of a blade may correspond to aseparation in the airspeed of the rotorcraft 10 that maintains aconstant advancing blade Mach number above or below the value at acrossing 104 a, 104 b, 104 c.

In selected embodiments, an alternative method for effecting a crossing104 a, 104 b, 104 c may be employed. One such alternative method may bereferred to as a static transition. During a static transition, theairspeed of a rotorcraft 10 may be held constant by one or more engines48. However, the trimming (e.g., collective pitch, cyclic pitch, masttilt, etc.) of a corresponding rotor 40 may be manipulated to achievethe lower or higher rotational speed with a proper separation from thecritical speed. This method may be employed if, for whatever reason(e.g., air-traffic control situations), the airspeed of the rotorcraft10 must be maintained.

Referring to FIGS. 9-12, in selected embodiments, a hub 44 may rotateabout a mast 46. This rotation may be enabled by one or more mastbearings 105. A mast 46 may cooperate with a hub 44 to form a manifold.A flow of compressed air generated by one or more engines 48 may bedelivered to this manifold by a plenum 56 connected to the mast 46.After entering the manifold, the flow of compressed air may divide as itpasses through the various feathering spindles 64 and enters theinternal conduits 60 of the corresponding rotor blades 42.

A hub 44, feathering spindle 64, and rotor blade 42 arrangement inaccordance with the present invention may provide an assembly thatremains suitably rigid in the desired directions or degrees of freedomthroughout the design temperature and load ranges. Accordingly, the hub44, feathering spindle 64, and rotor blade 42 arrangement may ensurethat the rotor 40 meets the flapping, lead-lag, and torsional stiffnessrequirements throughout the flight envelope.

In selected embodiments, a mast 46 may comprise or be connected to abase 110. A base 110 may form part of a tilt mechanism 82. Accordingly,a base 110 may comprise a platform or other structure that may bemanipulated (e.g., oriented or reoriented) to control the orientation ortilt of a mast 46. In certain embodiments, a base 110 may pivotablyengage one or more pivot mounts 112 connected to the rest of an airframe12. The pivot mounts 112 may be configured and positioned to permit abase 110 to pivot with respect thereto. For example, in the illustratedembodiment, the pivot mounts 112 enable a base 110 (and correspondingmast 46) to pivot about an axis 114 extending laterally with respect toan airframe 12.

An actuator 98 or actuation system 98 may control and motivate thepivoting of a base 110. In selected embodiments, an actuation system 98may include a motor 116 (e.g., pancake motor) controlling rotation of adrive shaft 118. Connected to a drive shaft 118 may be one or more wormdrives 120. Each worm drive 120 may engage a corresponding threadedmember 122. Accordingly, rotation by a motor 116 of a drive shaft 118 ina first direction may cause a worm drive 120 to advance along acorresponding member 122. Conversely, rotation by a motor 116 of a driveshaft 118 in a second direction, opposite to the first direction, maycause a worm drive 120 to retreat along a corresponding threaded member122.

Motion along a threaded member 122 may be used to pivot a base 110 withrespect to one or more pivot mounts 112. For example, in selectedembodiments, a first end of each threaded member 122 may be pivotablyconnected to an actuator mount 124 connected to the rest of an airframe12. Each threaded member 122 may extend from a corresponding actuatormount 124 to engage a corresponding worm drive 120. In selectedembodiments, a threaded member 122 may extend through an aperture 126 inthe base 110 to engage a worm drive 120. The aperture 126 may beoversized to accommodate the full range of motion of the threaded member122 therewithin.

In selected embodiments, each worm drive 120 may be connected to thebase 110. Accordingly, motion of a worm drive 120 along a threadedmember 122 may result in pivoting of a base 110 about one or more pivotmounts 112. A worm drive 120 may be connected to a base 110 in anysuitable manner. As a worm drive 120 travels along a threaded member 122and a base 110 pivots, an orientation of the threaded member 122 withrespect to the base 110 may change. Accordingly, in selectedembodiments, each worm drive 120 may pivotably connect to the base 110.This may enable a worm drive 120 to follow a corresponding threadedmember 122 as the threaded member 122 changes its orientation withrespect to the base 110.

In certain embodiments, a frame 128 may extend to connect various wormdrives 120 together. This may ensure that the worm drives 120 all pivotin concert, in a uniform manner with respect to a corresponding base110. A frame 128 may also provide a location for securing a motor 116 ina fixed relationship with respect to the worm drives 120 driven thereby.

Referring to FIG. 13, in selected embodiments, a tilt mechanism 82 maysupport pivoting of a mast 46 about multiple axes. For example, inaddition to supporting pivoting about an axis 114 extending laterally, atilt mechanism 82 may support pivoting of a mast 46 about an axis 130extending longitudinally with respect to an airframe 12. A tiltmechanism 82 may accomplish this in any suitable manner.

In selected embodiments, a tilt mechanism 82 may use layers 132 tosupport pivoting of a mast 46 about multiple axes 114, 130. A firstlayer 132 a may include a base 110 a, pivot mounts 112 a, motor 116 a,drive shaft 118, one or more worm drives 120 a, actuator mounts 124 a,frame 128 a, etc. supporting pivoting of a mast 46 about a first axis(e.g., an axis 114 extending laterally). A second layer 132 b mayinclude a base 110 b, pivot mounts 112 b, motor 116 b, drive shaft 118,one or more worm drives 120 b, actuator mounts 124 b, frame 128 b, etc.supporting pivoting of a mast 46 about a second axis (e.g., an axis 130extending longitudinally).

A mast 46 may comprise or connect to a base 110 a of a first layer 132a. Rather than connecting directly to the rest of an airframe 12, thepivot mounts 112 a and actuator mounts 124 a of the first layer 132 amay connect directly to a base 110 b of a second layer 132 b.Accordingly, the pivot mounts 112 b and actuator mounts 124 b of thesecond layer 132 b may be those that connect directly to the rest of anairframe 12. Thus layers 132 may be connected to other layers 132 inorder to move components with respect to one another in modes or degreesof freedom provided by the layers.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrative,and not restrictive. The scope of the invention is, therefore, indicatedby the appended claims, rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed and desired to be secured by United States LettersPatent is:
 1. A method comprising: flying a rotorcraft comprising anairframe, a propulsion source, a rotor operably connected to rotate withrespect to the airframe, a mast extending to connect the rotor to theairframe, a tilt mechanism operable to tilt the rotor with respect tothe airframe, and a computer system operably connected to detectoperational characteristics of the rotorcraft and effect controlthereof; identifying, by the computer system, during the flying, (a) animpending crossing of a natural frequency of the rotor; issuing in realtime, by the computer system, in response to the identifying (a), atleast one command to the tilt mechanism; and reorienting in real time,by the tilt mechanism, in response to the at least one command, the mastwith respect to the airframe while also adjusting at least one ofcollective pitch and cyclic pitch effective to increase speed at whichthe natural frequency is crossed.
 2. The method of claim 1, wherein thereorienting comprises pivoting the mast with respect to the airframefrom a first position to a second position.
 3. The method of claim 1,wherein the flying comprises sustained autorotative flight.
 4. Themethod of claim 1, wherein the flying comprises sustained autorotativeflight at an aircraft airspeed of at least 200 mph.
 5. The method ofclaim 1, further comprising reducing, as a result of the reorienting,the at least one flapping load.
 6. The method of claim 1, furthercomprising maintaining, as a result of the reorienting, the at least oneflapping load at substantially zero.
 7. The method of claim 1, whereinthe reorienting comprises pivoting the mast with respect to the airframeabout an axis extending laterally with respect to the airframe.
 8. Themethod of claim 1, wherein the reorienting comprises pivoting the mastwith respect to the airframe about an axis extending longitudinally withrespect to the airframe.
 9. The method of claim 1, wherein the rotor hasa ratio of a first and lowest in plane natural frequency to a rate ofrotor revolutions per minute that is greater than one.
 10. The method ofclaim 1, wherein the airframe further comprises a fixed wing.
 11. Amethod comprising: obtaining a rotorcraft comprising an airframe, arotor having an axis of rotation and being operably connected to rotatewith respect to the airframe, a mast extending to connect the rotor tothe airframe, a tilt mechanism connected to effect a change inorientation of the axis of rotation with respect to the airframe, atleast one sensor operably connected to detect a characteristic of therotor, and a computer system operably connected to the at least onesensor; flying the rotorcraft in sustained autorotation; identifying, bythe computer system, (a) an impending crossing of a natural frequency ofthe rotor due to slowing of the rotor; issuing in real time, by thecomputer system, in response to the identifying (a), a first command tothe tilt mechanism; and reorienting in real time, by the tilt mechanism,in response to the first command, the mast with respect to the airframe,such that the mast is tilted further forward than required to maintainthe rotor in steady state autorotation; slowing, due at least in part tothe reorienting, the rotational speed of the rotor; and crossing,subsequent to initiating the changing, the natural frequency of therotor by slowing the rotor to below the natural frequency.
 12. Themethod of claim 11, further comprising maintaining during the flying, asa result of the reorienting, the at least one flapping load atsubstantially zero.
 13. The method of claim 12, wherein the rotorprovides a value greater than one for a ratio of a first, in-plane,natural frequency to a rate of rotor revolutions per minute.
 14. Themethod of claim 13, wherein the airframe further comprises at least onefixed wing.
 15. The method of claim 14, wherein the flying furthercomprises supporting, by the at least one fixed wing, a majority of theweight of the rotorcraft.
 16. The method of claim 15, wherein thereorienting comprises pivoting the mast with respect to the airframeabout an axis extending laterally with respect to the airframe.
 17. Themethod of claim 16, wherein the reorienting further comprises pivotingthe mast with respect to the airframe about an axis extendinglongitudinally with respect to the airframe.
 18. The method of claim 11,farther comprising: identifying, by the computer system, (b) animpending crossing of a natural frequency of the rotor due toacceleration of the rotor; issuing in real time, by the computer system,in response to the identifying (b), a second command to the tiltmechanism; and reorienting in real time, by the tilt mechanism, inresponse to the second command, the mast with respect to the airframe,such that the mast is tilted further aft than required to maintain therotor in steady state autorotation; increasing, due at least in part tothe reorienting, the rotational speed of the rotor; and crossing,subsequent to initiating the changing, the natural frequency of therotor by accelerating the rotor to above the natural frequency.
 19. Arotorcraft comprising: an airframe; a rotor, having an axis of rotationand being operably connected to rotate with respect to the airframe; amast extending to connect the rotor to the airframe; a tilt mechanismselectively reorienting the mast with respect to the airframe; acomputer system programmed to identify in flight impending crossings ofnatural frequencies of the rotor; and the computer system, programmedfurther programmed to issue, in response to an identification of animpending crossing of a natural frequency of the rotor, the at least onecommand instructing the tilt mechanism to reorient the mast with respectto the airframe; wherein the computer system is further programmed to:identify that crossing of one of the natural frequencies of the rotor isnot impending; in response to identifying that crossing of one of thenatural frequencies of the rotor is not impending, controlling tilt ofthe mast effective to achieve a desired lift to drag ratio according toan aeromechanical model of the rotorcraft.
 20. The rotorcraft of claim19, wherein the rotor provides a value greater than one for a ratio of afirst and lowest, in-plane, natural frequency to a rate of rotorrevolutions per minute.